Technical Abstract:
Fusarium oxysporum Schlecht. is one of the most successful plant pathogens as reflected in the abundance and diversity of the 150 host-specific formae speciales described for this vascular wilt pathogen. Forma specialis are distinguished by unique host ranges and may consist of one or more vegetative compatibility groups (VCGs) and pathogenic races. Considerable genetic diversity within pathogenic and putatively nonpathogenic strains of F. oxysporum has been documented in numerous studies. We have generated DNA sequences from the mitochondrial small subunit (mtSSU) ribosomal DNA and translation elongation factor EF-1 alpha introns and exons to infer phylogenetic relationships of 332 strains within the F. oxysporum complex and to investigate its near relatives. Although the F. oxysporum complex is strongly supported as monophyletic, several F. oxysporum-like strains representing phylogenetically distinct species were nested outside of this complex. Therefore, strains identifie as F. oxysporum morphologically may exhibit either a para- or polyphyletic set of relationships. Within the F. oxysporum complex, four clades were resolved comprising the 332 ingroup strains sequenced. Approximately 80% of the formae speciales sequenced that have two or more VCGs appear to be either para- or polyphyletic. Ongoing studies are directed at extending this database to every forma speciales and VCG and towards developing a robust phylogeographic hypothesis for this complex. In addition, plans are in progress to post the aligned F. oxysporum DNA sequence database on the NCAUR Web site (http://nrrl.ncaur.usda.gov) by the end of 1999 as an electronically portable diagnostic tool for fungal biologists interested in determing the phylogenetic placement of isolated strains in this complex.